(1) Field of the Invention
The present invention relates to electronic circuits. In particular, the present invention relates to reducing body effect in electronic circuits.
(2) Background Information
The body effect is a well-known principle in MOS transistors--the threshold voltage of a Metal Oxide Semiconductor (MOS) transistor varies in accordance with variations in the source-to-bulk voltage of such transistor. In a source follower circuit this effect may cause non-linearity in the output voltage V.sub.out, as V.sub.out =V.sub.in -V.sub.TN, and the threshold voltage V.sub.T increases as the output voltage V.sub.out increases.
The body effect and its influence upon electronic circuits is hereinafter generally explained in connection with a basic source follower circuit 100 illustrated in FIG. 1a and the voltage characteristics of this circuit illustrated in FIG. 1b. FIG. 1b illustrates ideal and real output/input voltage characteristics 106 and 108 respectively of the source follower circuit illustrated in FIG. 1a. Ideally, without the body effect, the output voltage is V.sub.out =V.sub.in -V.sub.TN -V.sub.overdrive. V.sub.overdrive is the voltage over the threshold voltage V.sub.TN that needs to be applied to a gate of a transistor to provide a bias current. However, when the body effect is accounted for V.sub.out is lower. Source follower circuit 100 includes an active device such as NMOS transistors 101 and a load device which may be a device with a fixed impedance such as transistor 102 biased at a predetermined voltage V.sub.BIAS. Transistor 101, (pull-up transistor) has a gate that receives signal (V.sub.in) and a drain coupled to a voltage power supply (VCC). Second NMOS transistor 102, (pull-down transistor) has a gate that receives a V.sub.BIAS signal that turns ON transistor 102. The second NMOS transistor 102 has a source connected to ground. A source of first NMOS transistor 101 and a drain of second NMOS transistor 102 are commonly coupled to output node 103. Bulks of first and second NMOS transistors are coupled to ground.
In theory, the voltage at output node 103 should be V.sub.out =V.sub.in -V.sub.TN as illustrated by ideal characteristic 106 of FIG. 1b, where V.sub.TN is the threshold voltage of NMOS transistor 101. However, a voltage difference between the voltage applied at the source of the first NMOS transistor 101 and the ground voltage, applied to a bulk of this transistor, causes the voltage at output node 103 to be lower than V.sub.in -V.sub.TN as illustrated by curve 108 of FIG. 1b that takes into account the body effect. The reason is that the body effect causes V.sub.TN to raise with the difference V.sub.SB between the voltages at the source and bulk of NMOS transistor 101. This is reflected in the formula V.sub.TN =V.sub.TO +.GAMMA.(.sqroot..vertline.-2.PHI..sub.F +V.sub.SB .vertline.-.sqroot.2.vertline..PHI..sub.F .vertline.) according to which, when V.sub.SB .noteq.0, V.sub.TN increases with V.sub.SB. In this formula, V.sub.TO is the threshold voltage when V.sub.SB =0, parameter .GAMMA.(gamma) is termed the body effect coefficient or body factor, and the function .PHI..sub.F is termed equilibrium electrostatic potential in the semiconductor of the transistor. Accordingly, when V.sub.TN raises due to body effect, the voltage V.sub.in-V.sub.TN at the output node is decreased. This causes a decline in output gain.
One application where the body effect is problematic is a multiplexed analog bus with a large number of imputs ports, where each input port is driven by a source-follower. Each input ports may include at least one MOS transistor coupled to the analog bus. The body effect may affect the voltage driven to the analog bus due to variation of V.sub.TN with V.sub.SB. Such variation causes the voltage driven to the analog bus to be smaller than the actual voltage that could be driven to the analog bus in the absence of the body effect. Therefore, the body effect may cause a decrease in the output range of the analog data bus with multiplexed inputs port. It is desirable to reduce the body effect at the inputs port of a multiplexed analog bus, thereby increasing the output range of an analog bus. Moreover, parameters may vary from transistor to transistor, requiring an extra matching parameter. Reducing the body-effect from the transistor may reduce the desirability of providing the extra matching parameter.